Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



I have a concrete wall that has indentations where a metal fence post sits now partly filling the void from a 4x4 cedar post that I had to remove. Around this indentations is another indentation 1.5" deep where 2x8 form boards were placed to hold the cedar posts when the wall was poured. These indentations are every 4 feet. I want to put a form over them and pour a cement filler to fill in the indentations and make the wall smooth for its length. What would be the best product to use?
- Wayne
Tuesday, December 6, 2016 at 9:48 PM

How can one repair a cement slab around a 4" pvc pipe for the water closet collar? Over the years the concrete has deteriorated and there is a 1" to 2" gap around the pipe which no longer allows anchoring the water closet collar. The slab is about 3" thick and is also the basement ceiling.
- Don
Saturday, December 3, 2016 at 5:12 PM

Mark, Sand Mix Topping and Bedding Mix can be placed from 2" down to 1/2" in thickness. You could use the Sand Mix to top coat the area combined with our Bonder and Fortifier. Keep in mind that since there are already cracks in the surface on the concrete you may see transfer cracking in the new product. There is no other way to address this without ripping out the existing slab.
- Lee-Technical Service
Wednesday, November 16, 2016 at 8:23 AM

Our front front porch has dropped about two inches. The porch has a crawl space that ranges between 2-4 feet.The porch has been secured( no more sinking) The surface lookes terrible with cracks and is uneven. What products should I use to repair?
- Mark
Tuesday, November 15, 2016 at 4:12 PM

Eddy, unfortunately the paint will have to be completely removed before you could put a top coat over the existing slab. A bonding agent would not compensate for this.
- Lee-Technical Service
Tuesday, November 15, 2016 at 11:11 AM

Tim, it is not possible to use Top'n Bond on asphalt. You will need to consult a hot patch company to resurface your asphalt.
- Lee-Technical Service
Tuesday, November 15, 2016 at 11:09 AM

Maria, it is possible. But we would have no idea of the extent that it could be. We would recommend that you have someone look at it to access whether or not that it needs to be repaired. The repair material will need to be an overhead type of product. The depth of the repair would also determine the type of product.
- Lee-Technical Service
Tuesday, November 15, 2016 at 11:08 AM

I have a patio with painted concrete. I am going to extend and raise the concrete, putting concrete on top of the existing painted concrete. Do i need to do any kind of prep to the painted concrete to assure bonding?
- Eddy
Tuesday, November 8, 2016 at 12:01 PM

Is it possible to use top and bond on a blacktop [asphalt] garage floor?
- Tim
Thursday, November 3, 2016 at 1:04 PM

there was mold on concrete cold cellar ceiling and the people who removed it also chiseled out some parts of the concrete. the ceiling is the floor of the front porch. could this now cause some structural damage. how do we fill in those holes left behind to get a smooth surface?
- Maria Leach
Sunday, October 16, 2016 at 4:00 PM



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